The present invention has utility in applications requiring the conversion of an analog signal into a digital signal, for example for computer sensing of analog information in an automotive control system. To further illustrate, in an automotive engine control system, a microcomputer requires analog signal information from various transducers to be converted into digital signal information before it can be processed by the microcomputer. Examples of such analog signal information are the outputs of sensors for manifold pressure, oxygen, rotational speed, operator input, battery voltage, anti-knock, etc.
In a typical automotive application, many different analog signals need to be converted. It is known to provide an A/D converter module with a number of channel inputs, each responsive to a corresponding analog signal, and a multiplexer for gating the analog signals into the A/D circuit for conversion into digital signals. The optimum number of channel inputs depends upon the particular application for which the A/D converter is used.
There is a significant need in this technology for a way to increase the number of analog signals that can be converted, while maintaining the number of A/D converter I/O pins at a minimum. There is also a need to provide A/D converter circuits that can accommodate a wide range of number of analog inputs, depending upon the particular application.
It is known to provide one or more external multiplexer (MUX) IC's for an A/D converter module. In this way, the number of analog signals that can be sampled and converted can be increased.
In addition, the use of external MUX IC's permits a plurality of analog signals to be terminated nearer their sources, thereby minimizing the number of cables or conductors between the analog sources and the A/D converter module. The attendant reduction in weight, volume, and shielding has particularly important advantages for vehicle applications.
Each MUX external to an A/D converter module may be responsive to, for example, eight analog inputs. Under software control, involving the central processing unit (CPU) of the A/D converter system, individual MUX's may be addressed by the CPU, and one of the eight analog values may be input into the converter for conversion to a corresponding digital value. However, this system has the disadvantage of requiring the involvement of the system CPU software.
Thus there is a significant need to provide an A/D converter system in which external MUX IC's can be utilized without unduly burdening the system CPU's software operation.